Digital printing machines can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art. The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format.
The output sheet can be printed on one side only, known as simplex, or on both sides of the sheet, known as duplex printing. In order to duplex print, the sheet is fed through a marking engine to print on the first side, then the sheet is inverted and fed through the marking engine a second time to print on the reverse side. The apparatus that turns the sheet over is called an inverter.
FIG. 1 shows a state-of-the-art inkjet digital printing machine 20. Printer 20 includes a marking module or engine 22 having an ink jet print head or multiple print heads 23, disposed centrally on the marking engine 22, and facing downward. Printer 20 has a media path 24 along which the media sheet 34 moves, and a media path entrance 26 where sheets are fed into the printer by a media sheet feeder (not shown). Printer 20 also has a media path exit 28 where sheets leave the printer and are fed into a finisher (not shown). Printer 20 has an inverter 30 to turn the sheet over for duplex printing. A media sheet 34 leaving the inverter 30 follows arrow 32 back to the marking engine 22 for printing on the reverse side. Arrows 26 and 28 also indicate the process path direction, which is downstream from entrance 26 toward exit 28.
In cut sheet printing devices, under certain conditions, the lead-edge of the paper can curl up and have potential for separating from the marking transport and contact the print head. A sheet with out-of-spec flatness can occur when a duplexed sheet has a heavy ink image on the trail edge of side 1, which then becomes the lead edge when inverted and curls towards Side 2. This is most severe when the paper is thin, and the cross-process direction image is parallel to the grain direction of the paper (Example: letter size paper, grain-long, long-edge-feed).
In direct-to-paper ink jet marking engines, an ink jet print head is mounted such that the face (where the ink nozzles are located) is mounted a fixed distance from the surface of the media. The gap is typically 1 mm or less. Because the paper curl height can be several millimeters, it poses a risk to the print head because it can hit the print head face plate when it passes through the nominally thin gap that the print heads are spaced from the media.
Ink jet print heads are very delicate and can easily be damaged if the face of the print head is contacted by the media which is passing nearby. The print heads are also very expensive. Thus, it is very important to minimize any risk of damaging these print heads.
Accordingly, there is a need to provide a print head protection device for inkjet printers that will detect media sheet curl and take remedial action to prevent print head damage.
There is a further need to provide a print head protection device for inkjet printers of the type described and that will match the high production rate of a digital printing machine.
There is a yet further need to provide a print head protection device for inkjet printers of the type described and that is mechanically simple and robust, thereby minimizing cost.